Sludge is produced during the biological treatment of wastewater. It is also produced through a variety of processes, such as the manufacture of pulp and paper. As shown in a conventional process of FIG. 1, wastewater W is typically fed into a reactor 10 containing a suspension of microorganisms, most of which are acclimated to grow on or otherwise decompose the dissolved organic material contained in the wastewater W.
A fraction of the microorganisms dies and settles out. These solids, along with other solid debris that settles out from the wastewater, constitute sludge S that must be disposed of. The sludge S is typically collected and treated with chemicals at step 20 that promote flocculation of the sludge particles. One example of such chemical treatment is through use of thickeners such as polymers or lime. [1] Mechanical thickening has also been reported although the process has not been commercialized.
The flocculated sludge FS is dewatered at step 30, using various dewatering devices well-known in the art, including, but not limited to, presses of various configurations, and centrifuges. The dewatered sludge, or final cake solids FCS, are typically in the range of about 15-50% consistencies (i.e. 85-50% water content).
The final cake solids FCS are disposed of at step 40 through means such as landfilling, burning, or landspreading. The water contained in the dewatered sludge is a deadload, and it is thus advantageous to reduce the water content of the final cake solids as far as possible.
An electro-acoustic process where the application of electric and acoustic fields consolidates the cake and increases final solids is known. [2] Ultrasonic energy has been shown to dewater fine-particle high-concentration suspensions such as slurries and sludges. [3]
Discharging a high-intensity spark underwater generates a sound wave, and it has recently been demonstrated that some applications that rely on acoustics can also be conducted with an underwater sparker. For example, trace levels of adhesives in process streams of mills that recycle paper can be oxidized by either ultrasonic radiation or sparking. [4-6] However, the spark application can be more economical in terms of both capital and operating costs. [6].
Underwater sparking has been applied to anaerobically digested sludge in an effort to reduce polymer use. [7] Limited pilot-scale success was achieved with low-conductivity sludge, but the spark source was of relatively low energy (0.25 J), and the application did not successfully scale up.
It would be beneficial to demonstrate that exposing biological sludge to underwater sparks of much higher energy (˜10 kJ) leads to acoustic agglomeration of the particles, which results increased cake solids, and faster settling.
Increasing the cake solids by even a relatively small amount would reduce the amount of water that must be disposed of together with the sludge solids. The rate of dewatering is also an important factor that affects the efficiency and cost of processing of the sludge. If the dewatering rate is slow, then the throughput will be low.
Therefore, there is a need for a system and method for increasing the solids content of dewatered sludge. There is also a need for a system and method for increasing the rate of sludge dewatering by altering the properties of the sludge.